Someone who commonly whines, complains, or looks at the bad side of things.

2nd grade teacher: "All right kids, today we're going to learn an important lesson: how to pack your rock into your crack pipe."
Johnny: "But Mrs. Stoner, my mommy told me drugs are bad."
Teacher: "Johnny, stop being such a negative nancy. You'll never get anywhere in life with that attitude."
Johnny: "But, my mommy said..."
Teacher: "Johnny, please shut the fuck up or I'll send you to time out and brutally murder your family."

The Urban Dictionary T-Shirt

A nickname given to a guy or a girl who always has a negative spin on things. Always brings the mood down with the doom and gloom analogies.

Jack: Way to go Billy...a grand slam!
Tom: God, it's about time he only struck out the last four at bats.
Jack: Just shush and quit being such a Negative Nancy and be thankful it wasn't a fifth strikeout, but it was a grand slam.